In vascular injury and atherogenesis different cell types are recruited to the injured vessel. Early on, circulating leukocytes, especially monocytes, are recruited to the vessel by chemoattractant signals. After adhering to the endothelial lining of vessels, monocytes migrate across the endothelium, where they differentiate into macrophages and ultimately become lipid-laden foam cells. Attractant signals also recruit VSMC from the medial to the intimal layer, where they proliferate and secrete extracellular matrix proteins, causing neointimal proliferation. In the injured blood vessel, platelets adhere to endothelium, to macrophages, and to exposed collagen. Platelets become activated, spread, and secrete cytokines and other inflammatory mediators. These mediators worsen vascular injury by recruiting and activating more leukocytes, by stimulating migration and proliferation of VSMC, and by recruiting more platelets. Collectively, these individual events promote vascular inflammation, neointimal proliferation, and thrombosis, causing occlusion of both native and stented blood vessels.
Cardiovascular disease leading to heart attack and stroke remains the leading cause of mortality and morbidity in the Western world (Libby 2002; Meadows and Bhatt 2007). Atherosclerosis is a progressive disease characterized by accumulation of inflammatory cells and vascular smooth muscle cells (VSMC) within the intima of injured blood vessels. The atherosclerotic plaque is made up of immune cells, including monocytes, macrophages, neutrophils, T lymphocytes, and VSMC which migrate from the media to the intima, where they proliferate and secrete extracellular matrix proteins. Platelets have also been implicated in the initiation of atherosclerotic lesion formation (Massberg, Brand et al. 2002; Huo, Schober et al. 2003; von Hundelshausen and Weber 2007). These events result in progressive narrowing of the vessel, allowing platelet aggregation and activation to ultimately form vascular occlusive thrombi, precipitating acute coronary syndromes and ischemic stroke (Gawaz, Langer et al. 2005).
Strategies that block recruitment to the intima of immune cells and VSMC are partially protective against vascular injury in both animal models and human patients. Inhibiting monocyte and VSMC recruitment to selected chemoattractants partially prevents atherosclerosis and its clinical manifestations, and simultaneous blockade of two chemotactic pathways confers additional, but not complete, benefit (Boring, Gosling et al. 1998; Gosling, Slaymaker et al. 1999; Libby 2002; Combadiere, Potteaux et al. 2003; Lesnik, Haskell et al. 2003; Saederup, Chan et al. 2008). Medical therapies that inhibit platelet activation and aggregation are a mainstay of treatment for patients at risk for cardiovascular events, making anti-platelet agents the most prescribed drugs worldwide (Meadows and Bhatt 2007). However, these agents also only provide partial protection against cardiovascular events.
Acute kidney injury (AKI) develops in 5% of hospitalised patients and leads to significant morbidity, mortality and financial costs (Brady and Singer 1995; Korkeila, Ruokonen et al. 2000; Bagshaw 2006). Fifty percent of cases of AKI result from ischaemia-reperfusion injury (IRI) (Thadhani, Pascual et al. 1996). Despite significant advances in understanding the cellular and molecular events that cause kidney IRI, specific therapy remains elusive and management is mainly supportive (Jo, Rosner et al. 2007).
The recruitment of circulating leukocytes, particularly neutrophils into the injured kidney is a key component of AKI caused by IRI (Okusa 2002). Once recruited to the injured kidney, the recruited leukocyte subsets promote and perpetuate the organ damage (Furuichi, Wada et al. 2003; Friedewald and Rabb 2004; Fiorina, Ansari et al. 2006). Therapies targeting different leukocyte subsets are partially effective in ameliorating the injury associated with AKI (Kelly, Williams et al. 1994; Singbartl, Green et al. 2000; Jo, Sung et al. 2006). However given the diversity of the recruiting signals and the cells recruited, it is unlikely that a therapy directed at a single leukocyte will be entirely effective (Salmela, Wramner et al. 1999). Early after reperfusion, platelets also adhere within capillaries of the vasa recta. After adhering, platelets become activated, spread, and release acute inflammatory mediators and pro-fibrotic growth factors that intensify kidney injury and scarring (Li, L and Okusa, M D 2006. Nat Clin Pract Nephr 2:432-444). Therapies that inhibit platelet function also partially, but not completely, ameliorate AKI (Singbartl 2000; Chintala M S et al 1994 JPET 271:1203-1208).
The Slit family of secreted proteins, together with their transmembrane receptor, Roundabout (Robo), act as repellents for migrating neurons and axons during development of the central nervous system (Kidd, Brose et al. 1998; Brose, Bland et al. 1999; Kidd, Bland et al. 1999). It has recently been appreciated that Slit and Robo are also expressed in mature organisms, and an isoform of Robo, Robo-1, has been detected on the surface of several cell types involved in vascular injury, IRI and atherogenesis, including VSMC, neutrophils and mononuclear leukocytes (Wu, Feng et al. 2001; Prasad, Fernandis et al. 2004; Liu, Hou et al. 2006; Prasad, Qamri et al. 2007; Tole, Mukovozov et al. 2009). Slit2 has been shown to interact with Robo-1 to prevent directional migration of these cells in response to diverse inflammatory chemoattractant cues both in vitro and in vivo (Wu, Feng et al. 2001; Guan, Zu et al. 2003; Kanellis, Garcia et al. 2004; Prasad, Fernandis et al. 2004; Liu, Hou et al. 2006; Prasad, Qamri et al. 2007; Tole, Mukovozov et al. 2009).